[ORC] Update lazyReexports to support aliases with different symbol names.

A bug in the existing implementation meant that lazyReexports would not work if
the aliased name differed from the alias's name, i.e. all lazy reexports had to
be of the form (lib1, name) -> (lib2, name). This patch fixes the issue by
capturing the alias's name in the NotifyResolved callback. To simplify this
capture, and the LazyCallThroughManager code in general, the NotifyResolved
callback is updated to use llvm::unique_function rather than a custom class.

No test case yet: This can only be tested at runtime, and the only in-tree
client (lli) always uses aliases with matching names. I will add a new LLJIT
example shortly that will directly test the lazyReexports API and the
non-trivial alias use case.